Macrosegregation is one parameter used to measure the properties of a finished product so as to determine its future usefulness. Changes in macrosegregation across commercial size castings, particularly aluminum alloy ingots produced in a continuous casting mold, make it difficult to maintain a particular concentration of alloying elements within specification throughout the entire cross section or thickness of a casting. The degree of macrosegregation in a casting will be influenced to a large extent by the cast thickness, by alloying elements and their concentrations, and by the process used in casting the ingots. Laboratory experimentation has shown that as the thickness or diameter of a cast ingot increases, the degree of macrosegregation will also increase. This is a direct result of unavoidable variations in the solidification brought about by varying heat extraction rates from location to location within the ingot cross section and by convective forces in the unsolidified portion of the ingot. In a continuous casting process such as a direct chill casting process for an aluminum alloy ingot, it is customary to have a liquid zone followed by a mushy zone which in turn is followed by a solid zone, all of which are arranged in a vertical orientation. The mold is water cooled and therefore the outer surface tends to solidify prior to the central portion adjacent to the longitudinal centerline. Dendrites which are lean in eutectic elements tend to grow about the periphery of the liquid zone. The presence of strong convection currents within the liquid zone causes the tips of the dendrites to detach and be carried by the convection currents to the center of the liquid zone. As the dendrites move toward the center, they grow isothermally within the thermal boundary layer and are finally frozen adjacent to the longitudinal centerline. Since the dendrites are lean in eutectic material, they cause the final product to exhibit a low concentration of the eutectic elements in its central portion. This low concentration causes a large variation in macrosegregation of the ingot itself which is undesirable for the reasons stated above.
Up until now, no one has been able to reduce macrosegregation adjacent to a longitudinal centerline of a commercial size ingot. U.S. Pat. No. 3,842,895, issued to Mehrabian et al, attempted to reduce channel segregation in the mushy zone of a casting by controlling heat flow during solidification. Mehrabian teaches that this can be accomplished by controlling the depth of the mushy zone by using a transverse magnetic field of the order of 2,000 gauss. By increasing the temperature of the liquid melt around the region of the liquidus-isotherm, while maintaining substantially the velocity of the solidus-isotherm, the depth of the mushy zone is supposedly kept at a desired level. Mehrabian does not teach reduction of macrosegregation along the longitudinal centerline of a casting by controlling convection currents with a mechanical damper positioned within the liquid pool of the mold. Three other patents, German Pat. No. 1,583,602, Russian Pat. No. 187,255 and Japanese Pat. No. 58-163,566, also disclose using an electromagnetic field to suppress the convection flow of a molten metal. However, none describe the use of non-electromagnetic means to control convection currents within the liquid pool of the mold.
Now a process and apparatus have been invented which reduces macrosegregation adjacent to a longitudinal centerline of a solidified casting.